The invention relates to a screen-printing device.
A device of this nature is generally known. Flat-bed screen-printing devices and rotary screen-printing devices spring to mind. During a screen-printing process, a liquid printing medium, such as ink, paint or paste, is metered through a screen onto a substrate which is to be printed. The substrate is guided past one or more printing stations by means of conveyor means, for example comprising a number of rollers and/or a conveyor belt. Each printing station comprises a screen and a squeegee. Pressure medium is supplied and is printed through the screen by means of a squeegee. The substrate may comprise textile, paper, plastic, metal and the like. If a new design is desired after a printing process, the screens are changed. Since the screens and squeegees used have been in contact with the printing medium, they have to be cleaned. If, following a printing process, the same design, but, a different colour, is desired, the screen is removed from the device and taken to a separate washing chamber, where it is cleaned and then replaced.
A drawback of the known screen-printing devices is that there is a considerable risk that, during changing of a screen, the substrate which is to be printed or has already been printed and is located beneath the screen, or even the device itself, may become dirty or damaged. The risk of the substrate or device becoming dirty is high in particular if screens and squeegees which have just been cleaned are placed in the device, since washing liquid can keep on dripping downwards. Messing of printing medium, washing liquid and the like on a substrate which is to be printed or has been printed, during production, leads to the substrate being rejected or labelled as seconds.
In Research Disclosure No. 37350 (May 1995) a rotary screen printing machine is disclosed, comprising a kind of roller blind which is to be rolled off from a roll and placed between a printing blanket and a set of several screens. The blind is destined for the removal of the set of screens from the machine, washing them in a separate washing machine and putting them back, without the risk of spilling paste onto a substrate.
In addition to the blind being difficult to clean, a major disadvantage of this known machine is that the removal, washing and replacement of the screens takes up considerable manpower and operating time.
Rotary screen-printing devices in which screens are washed on the device itself are also known. By way of example, EP-B-0,364,918 has disclosed a screen wherein a washing-liquid feed chamber is arranged inside the screen. The chamber has a plurality of spray nozzles which are directed at the squeegee and the ends of the screen. Discharge of residues of printing medium and washing liquid take place over the substrate lying below the screen, a piece of cloth which is sewn in between or over the substrate-conveying means.
A drawback of this is that substrate is wasted or has to be resewn, or that the substrate-conveying means have to be cleaned after the liquids have drained off them.
The object of the invention is to overcome the abovementioned drawbacks and, in particular to provide a screen-printing device which can be employed flexibly and efficiently.
According to the invention, this object is achieved by means of a screen-printing device according to claim 1. In this device, a screen and substrate-conveying means can be moved with respect to one another between a printing position and a free position, by suitably controlling a movement mechanism. In the printing position, the screen bears against a substrate which is guided past it. In the free position, the screen is located at a short distance from the substrate and/or the substrate-conveying means. A movable shield is provided at the location of the screen. The shield can move between an at-rest position and a shielding position. In the shielding position, the shield is located between the screen and the substrate or the substrate-conveying means and forms a barrier between them. For this purpose, the screen has to be in the free position with respect to the substrate-conveying means. In the at-rest position, the shield is not situated between the substrate and the screen, so that a printing process is possible. The device comprises discharge means for discharging liquid and contaminants which are deposited on a shield which has been placed in the shielding position.
Thus it is advantageously possible, according to the invention, to change and/or wash screens and/or squeegees without the risk of liquids, such as washing liquids or residual printing medium, being deposited on the substrate which is to be printed or has already been printed or on the device itself. In the shielding position, the shield collects all the liquids which run down and discharges them by means of the discharge means. The shield may optionally be placed in the shielding position as soon as a printing station is not used. This ensures that residual printing medium which leaks out-of the screen is collected. In order to prevent the screen from drying out, it may be moistened during prolonged rest periods. Any moistening liquid which in the process is deposited on the shield can be discharged by means of the discharge means. Advantageously, the shield according to the invention may also be used as a guide or insertion support when changing screens on the screen-printing device. The primary function of the shield is to collect and discharge washing liquid if a screen is being cleaned on the device itself. It is thus possible to wash screens and/or squeegees without them first having to be removed from the device and without there being any possibility of contamination to other parts of the device. This increases the efficiency of the device considerably, in particular when a printing process which uses the same design but different colours has to be carried out repeatedly. The work required is then reduced substantially, and considerable time is saved.
The invention provides a screen-printing device which is able to operate very cleanly and reliably. In the case of a plurality of printing stations, it is possible to provide one shield per printing station or per group of printing stations which lie next to one another.
In a preferred embodiment, a washing-liquid feed is provided on the screen-printing device itself. The washing-liquid feed is arranged in such a manner that a screen can be cleaned in the free position. However, it is also possible to clean the screens in the free position with an external washing-liquid feed which does not form an integral part of the screen-printing device.
A further object of the invention is to provide a screen-printing device with a plurality of printing stations which are located one behind the other and with which it is possible to print at one or more printing stations which can be selected as desired, while at the same time, and on the same device, screens and/or squeegees can be changed and/or washed at one or more other desired printing stations without there being any risk of contamination or damage to the substrate which is to be printed or has already been printed or to the screen-printing device itself.
According to the invention, this object is achieved by means of a screen-printing device according to claim 3. A separately operable movement mechanism and a separately operable shield is provided for each separate printing station. It is thus possible to perform highly flexible printing with any desired number of printing stations, while the printing stations which are not in use can be shielded. This prevents printing medium or other liquids being able to pass from these printing stations which are not in use onto the substrate which is to be printed or has already been printed or the device itself. It is advantageously possible, due to the possibility of separate operation, to carry out printing in a continuous process using any desired number of printing stations, while for the printing stations which are not in use the preparations for a subsequent printing process, in which a new design or a different colour is to be printed, can already be ongoing. While a printing process is in full swing, the printing stations which are not being used can be reliably shielded from the printing process, and the screens and/or squeegees can be cleaned, kept moist and the like without them first having to be removed from the device. The liquids which are deposited on the separate shields are discharged by means of discharge means which adjoin these shields.
Further preferred embodiments of the invention are defined in the other claims.